How to diagnose and fix a one headlight dimmer than the other issue
If you have a situation with one headlight dimmer than the other, the problem can be caused by several different factors, from a cloudy headlight lens or old headlight bulb, to a wiring problem. Here’s an easy list of items to check to find the source of the problem.
Cloudy headlight lenses?
Compare the lens condition of each headlight.
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When the headlight clear coat deteriorates, the plastic lens oxidizes and turns cloudy. A cloudy lens can reduce light output by up to 70%. If your headlight lens is cloudy, you can recondition it yourself with a headlight restoration kit. See this post for instructions.
Is the headlight bulb worn?
As headlight bulbs accumulate burn hours, the filament vaporizes and leaves a grey or brown metallic deposit on the inside of the headlight capsule. The deposit acts like a light filter and can dramatically reduce the light output from that bulb, causing the one headlight dimmer than the other issue.
Remove the headlight bulbs and compare them for deposits. If the dim headlight has evidence of these deposits, replace it and then see if one headlight is dimmer than the other. If so, proceed to the next step.
Check and clean the headlight ground connection
Many drivers think that a dim headlight is caused by a poor power connection. That’s rarely the case. It’s far more likely that vehicle is delivering enough power, but the ground connection is corroded, therby reducing light output.
Depending on the year, make or model, the headlights may have a common ground for both the left and right headlights, or each headlight may have its own ground. See the wiring diagrams below.
Read more: Headlight covers – Wikipedia
You can test the ground connections for voltage drop due to corrosion. But that takes longer than simply cleaning the ground connection. To clean the connection, simply remove the bolt, sand off the corrosion, coat both the ring terminal and the body sheet metal with a light film of dielectric grease and reassemble. Then test the headlights for uniform brightness. If the one headlight is dimmer problem disappears, you’ve solve the problem.
Finally, check the headlight connector
Some carmakers have reduced the wire gauge to the headlight connector and that has caused the plastic connector to melt. The result is poor conductivity. If you have a melted connector, buy a new headlight pigtail at any auto parts store and splice it in place of the melted unit.